What is a transistor?
A transistor is a type of semiconductor device that can be used to conduct and insulate electric
current or voltage. A transistor basically acts as a switch and an amplifier. In simple words, we can
say that a transistor is a miniature device that is used to control or regulate the flow of electronic
signals.
Parts of a Transistor
A typical transistor is composed of three layers of semiconductor materials or, more specifically,
terminals which help to make a connection to an external circuit and carry the current. A voltage or
current that is applied to any one pair of the terminals of a transistor controls the current through the
other pair of terminals. There are three terminals for a transistor. They are listed below:
Base: This is used to activate the transistor.
Collector: It is the positive lead of the transistor.
Emitter: It is the negative lead of the transistor.
Well, the very basic working principle of a transistor is based on controlling the flow of current through
one channel by varying the intensity of a smaller current that is flowing through a second channel.
Types of Transistors
There are mainly two types of transistors, based on how they are used in a circuit.
Bipolar Junction Transistor (BJT)
The three terminals of BJT are the base, emitter and collector. A very small current flowing between
the base and emitter can control a larger flow of current between the collector and emitter terminal.
Furthermore, there are two types of BJT, and they include:
P-N-P Transistor: It is a type of BJT where one n-type material is introduced or placed between
two p-type materials. In such a configuration, the device will control the flow of current. PNP
transistor consists of 2 crystal diodes which are connected in series. The right side and left
side of the diodes are known as the collector-base diode and emitter-base diode, respectively.
N-P-N Transistor: In this transistor, we will find one p-type material that is present between two
n-type materials. N-P-N transistor is basically used to amplify weak signals to strong signals. In
an NPN transistor, the electrons move from the emitter to the collector region, resulting in the
formation of current in the transistor. This transistor is widely used in the circuit.
There are three types of configuration, which are common base (CB), common collector (CC) and
common emitter (CE).
In common base (CB) configuration, the base terminal of the transistor is common between input and
output terminals.
In common collector (CC) configuration, the collector terminals are common between the input and
output terminals.
In common emitter (CE) configuration, the emitter terminal is common between the input and the
output terminals.
How Do Transistors Work?
Let us look at the working of transistors. We know that BJT consists of three terminals (Emitter, Base
and Collector). It is a current-driven device where two P-N junctions exist within a BJT.
One P-N junction exists between the emitter and base region, and the second junction exists between
the collector and base region. A very small amount of current flow through the emitter to the base can
control a reasonably large amount of current flow through the device from the emitter to the collector.
In the usual operation of BJT, the base-emitter junction is forward-biased, and the base-collector
junction is reverse-biased. When a current flows through the base-emitter junction, the current will
flow in the collector circuit.
In order to explain the working of the transistor, let us take an example of an NPN transistor. The
same principles are used for the PNP transistor, except that the current carriers are holes, and the
voltages are reversed.
Operation of NPN Transistor
The emitter of the NPN device is made of n-type material; hence, the majority of carriers are
electrons. When the base-emitter junction is forward-biased, the electrons will move from the n-type
region towards the p-type region, and the minority carrier holes move towards the n-type region.
When they meet each other, they will combine, enabling a current to flow across the junction. When
the junction is reverse-biased, the holes and electrons move away from the junction, and now, the
depletion region forms between the two areas and no current will flow through it.
When a current flows between the base and emitter, the electrons will leave the emitter and flow into
the base, as shown above. Normally, the electrons will combine when they reach the depletion
region.
But the doping level in this region is very low, and the base is also very thin. This means that most of
the electrons are able to travel across the region without recombining with holes. As a result, the
electrons will drift towards the collector.
